1. Field of the Invention
The present invention is directed to inhibiting popcorn polymer formation in organic material, and likewise to inhibiting further growth of popcorn polymer seeds or deposits already in existence. The desired result is effected by treatment of the organic material, or the seeds or deposits, with an amount of one or more mercaptans sufficient to inhibit popcorn polymer growth.
2. Description of Background and Other Information
Popcorn polymers are known to form from all manner of organic material, particularly olefinically unsaturated monomers, including olefins and diolefins; especially susceptible are the conjugated diolefins, e.g., butadiene and isoprene, and vinyl compounds, e.g., styrenes and acrylates. Known as popcorn polymers because they resemble popped corn, these polymers are also referred to in the art as sponge polymers, granular polymers, cauliflower-like polymers, nodular polymers, fluffy polymers, proliferous polymers, and crusty polymers.
Popcorn polymer has been considered to occur from spontaneous monomer polymerization. It can occur in both liquid phase and vapor phase, and at any stage of use or handling of the monomer, e.g., recovery, separation, manufacturing, purification, storage, etc. High concentrations of monomer are particularly advantageous for its formation.
Specifically, it appears that the presence of one or more initiators--e.g., water, oxygen, hydrogen, peroxide--results in the formation of popcorn polymer "seeds" in the organic material. The seeds themselves then perpetuate polymerization, without further requiring an initiator and/or a crosslinking agent; they serve as sites for further polymerization.
As the particular mechanism, it is believed that monomer diffuses through the surface of the growing polymer mass, and is added to the polymer at the center thereof. For this reason, such polymerization is referred to as occurring "from the inside out."
Consequently, there is continued incorporation of monomer into the polymer phase, leading to buildup of the popcorn polymer. The result is a hard polymeric foulant, which can cause serious equipment and safety concerns if left unchecked.
A particular problem attendant upon popcorn polymer formation is its extreme resistance to deactivation, once present in a system. Some of the seeds become attached to the processing and handling equipment, and cannot be readily removed by mechanical means; moreover, being insoluble in most common solvents, they are virtually impossible to wash out by use of such solvents.
Even after equipment and storage facilities have been cleaned thoroughly, residual particles of popcorn polymer remain, and promote unwanted polymer growth. Trace particles remaining in the equipment will stay active for long periods without the presence of monomer, and serve to initiate polymerization when once again contacted therewith.
Different inhibitors are known for use against popcorn polymer formation. Examples of these are the following: t-butylcatechol; sodium nitrite, as disclosed in LIU, "Plugging-Up of Equipment by Self-Polymerization Butadiene Production and Its Prevention," China Synthetic Rubber Industry, 11(5) 357-360 (1988); N,N-dialkylhydroxylamines, as disclosed in TOKAI ELECTRO-CHEMICAL CO., Japanese Kokai No. 66,223,003, as well as in LIU et al., "Determination of Traces of Diethylhydroxylamine Inhibitor in C.sub.5 Fraction by Gas Chromatography," China Synthetic Rubber Industry, 12(6), 408-410 (1989), and in ALBERT, U.S. Pat. No. 3,148,225, the latter of these also referring to nitrites, nitroso compounds, NO.sub.2, N.sub.2 O.sub.3, phenolic compounds, sulfur, aromatic amines, and hydroxylamine as being known in the prior art; trialkylamine oxides, as also disclosed in TOKAI ELECTRO-CHEMICAL CO.; N-hydroxymorpholine, used in conjunction with N,N-dialkylhydroxylamines, as disclosed in CASE et al., U.S. Pat. No. 3,265,752, or in conjunction with N-hydroxypiperidine, as disclosed in McCOY et al., U.S. Pat. No. 3,265,751; adducts of phenols and hydroxylamines, as disclosed in ALBERT et al., U.S. Pat. No. 3,493,063; reaction products of nitrous acid and 1,3-dichlorobutene-2 or diisobutylene, as disclosed in BENJAMINS, U.S. Pat. No. 3,560,577, which also refers to nitrogen dioxide, the addition product of 1,3-dichloro-2-butene and nitrogen dioxide, and ion-exchange resin containing nitrite ions, as being known in the prior art; butyraldoxime, as disclosed in KEOWN, U.S. Pat. No. 3,560,577; and nitrogen tetroxide-diisobutylene addition products, as disclosed in COLBERT, U.S. Pat. No. 3,175,012.
Further, sulfur-containing compounds, and even, specifically, hydrogen sulfide and certain mercaptans, are known as popcorn polymer inhibitors. HASKELL, U.S. Pat. No. 4,404,413, not only discloses elemental phosphorous and carbon disulfide, and additionally mentions ethyl disulfide as being known in the prior art, but, yet further, refers to hydrogen sulfide, and to ethane-, propane-, and hexane-thiol, also as being known for inhibiting popcorn polymer growth.
However, the HASKELL patent provides only limited discussion with respect to this matter. Various aspects of using hydrogen sulfide and mercaptans for inhibiting popcorn polymer growth are neither disclosed nor suggested in the prior art.
For instance, the only mercaptans discussed in HASKELL for inhibiting popcorn polymer formation are specific alkyl monothiols. There is no mention of alkyl monothiols in general, or of other types of aliphatic mercaptans, such as alkyl dithiols; particularly, there is no disclosure or suggestion of aromatic mercaptans in any form.
There is further no disclosure or suggestion, for any mercaptan, including those specific mercaptans listed in HASKELL, that popcorn polymer inhibition could be especially effective where the mercaptan inhibitor and the organic materials are similar with respect to structure or particular properties. As specific examples, there is no indication that such superior results are possible where both the mercaptan and the organic material are aromatic compounds, or where the mercaptan and the organic material have not only similar boiling points, but also vapor pressures which become similar as their respective boiling points are reached.
Also, not disclosed or suggested, for any mercaptan, or for hydrogen sulfide, are particular different methods of using such compounds as inhibitors. As an example, there is no mention of either continuously or intermittently adding the inhibitor to popcorn polymer-forming material. There is further no reference to treating actual popcorn polymer already formed, i.e., applying the inhibitor to the popcorn polymer itself, to prevent or retard further growth thereof.
It has been discovered that such uses of mercaptans and hydrogen sulfide, not previously disclosed or suggested, will inhibit popcorn polymer growth. It is further considered that, among the indicated newly discovered applications of mercaptans and hydrogen sulfide, use of particular such sulfur-containing compounds with particular organic materials, or popcorn polymers derived therefrom, and use of particular types of such sulfur-containing compounds with particular types of organic materials, or with popcorn polymers derived therefrom, will provide particularly effective results with respect to inhibiting growth (or further growth) of popcorn polymer.